Digital to analog signal converters usually employ a source of bit-parallel digital signal values to control the states of a set of switches which in turn control the application of a reference supply current or voltage to respective bit inputs of a resistance ladder network. The reference supply and network interact to combine the different bit signals of each digital sample with appropriate weighting to yield an analog signal sample with amplitude corresponding to the value of the digital signal sample.
A converter using a weighted resistor network and providing an output through an operational amplifier is shown in an article entitled "Digital-to-analog converter" by D. H. Sheingold, at pages 257-259 of MCGRAW-HILL ENCYCLOPEDIA OF SCIENCE & TECHNOLOGY, Fifth Edition, Vol. 4, 1982. The output appears as a voltage developed across a feedback resistor of the amplifier.
Another converter, employing the well-known R-2R resistor ladder network, is shown in an article entitled "31.6 Digital to analog converters (DAC)" at pages 31/10-31/11 in ELECTRONICS ENGINEER'S REFERENCE BOOK, Fifth Edition, 1983, Edited by F. F. Mazda. That converter provides current source connected transistors between digital-signal-controlled switches and the respective, ladder rung inputs. A reference current amplifier provides the necessary current, and the converter output is a current proportional to the digital sample value. Problems with linearity and, in the reference amplifier, settling time as well as the balance between stability and speed are noted in the article. Current sources are well-known to have compliance problems at the relatively high currents necessary for converter operations.
One problem often encountered is the need for amplification of a converter output before utilization. In high frequency systems, e.g. sample rates in the tens of megahertz range, of sufficient bandwidth and linearity for transmitting signals for, e.g., video display purposes, an amplifier generally is complex and therefore typically relatively expensive. If the converter is to drive the utilization circuit directly, a transmission line such as coaxial cable is usually required because it is not often convenient to locate the converter and utilization circuit immediately adjacent to one another. However, converter ladder networks typically have output resistances of the order of thousands of ohms and there are impedance matching difficulties in matching such high resistances to the characteristic impedance of transmission lines. One reason for the high output impedance level of the ladder network is to make its linearity of conversion over the full dynamic range independent of switch, or other transistor, effective resistances in circuits driving the ladder rungs.